University of WollongongResearch Online

Faculty of Engineering and Information Sciences -Papers: Part A Faculty of Engineering and Information Sciences

2017

Development of microencapsulated phase changematerial for solar thermal energy storageWeiguang SuUniversity of Nottingham, Qilu University of Technology

Jo DarkwaUniversity of Nottingham

Georgios KokogiannakisUniversity of Wollongong, gkg@uow.edu.au

Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library:research-pubs@uow.edu.au

Publication DetailsSu, W., Darkwa, J. & Kokogiannakis, G. (2017). Development of microencapsulated phase change material for solar thermal energystorage. Applied Thermal Engineering, 112 1205-1212.

Development of microencapsulated phase change material for solarthermal energy storage

AbstractIn this paper a novel microencapsulated phase change material (MF-3) has been developed and tested forsolar assisted hot water storage systems. Even though the morphology of the sample was affected by the typeof emulsifier used for fabrication it recorded the highest energy storage capacity of 126 kJ/kg withencapsulation efficiency of 97.4% as compared with other developed samples. For the purpose of assessing itsthermal effectiveness it was theoretically evaluated in a compacted fixed bed TES unit and found to be capableof achieving a higher energy storage density as well as relatively smaller physical storage size than water basedsystem. Despite the overall effective thermal conductivity being slightly less than water, its value was stillabout twice as high as most current PCM storage units. Experimental evaluation is therefore stronglyencouraged.

Keywordsthermal, solar, storage, material, energy, change, phase, microencapsulated, development

DisciplinesEngineering | Science and Technology Studies

Publication DetailsSu, W., Darkwa, J. & Kokogiannakis, G. (2017). Development of microencapsulated phase change material forsolar thermal energy storage. Applied Thermal Engineering, 112 1205-1212.

This journal article is available at Research Online: http://ro.uow.edu.au/eispapers/6317

1

Development of microencapsulated phase change

material for solar thermal energy storage

Weiguang Su1,a, Jo Darkwa2, Georgios Kokogiannakis3

1. School of Mechanical & Automotive Engineering, Qilu University of Technology, Jinan,

China

2. Faculty of Engineering, University of Nottingham, UK

3. Sustainable Buildings Research Centre, University of Wollongong, Australia

Nomenclature

, , the specific heat of water, core material and shell material

latent heat of PCM

enthalpy

, , the conductivity of water, paraffin and shell material

, , the weight percentage of water, core material and shell material

the weight percentages of MEPCM in TES unit

, , the volume percentage of water, shell material and core material

the temperature difference

the core material content of MEPCM

a Corresponding author. E-mail address: wgsuper@hotmail.com

2

1 Introduction

Solar thermal energy storage (TES) systems are considered to be among the commonest methods

of providing hot water or space heating services in buildings due to their relatively lower cost and

ease of operation [1-3]. For instance, a seasonal solar TES water tank was used to improve the

energy performance of district heating/cooling and hot water supply systems by Tanaka et al [4].

Furbo et al.[5] developed a smart solar TES tank for a small solar domestic hot water system,

which can be heated both by solar collectors and by means of an auxiliary energy supply system.

The investigations showed that the yearly thermal performance of the smart solar system was

5–35% higher than traditional system. However, the TES tanks do experience buoyancy

dependent stratification which tends to affect their heat transfer processes [6-8]. They also tend

to be relatively bulky and occupy more useable space[9]. To this end, Morrison et al. [8] have

investigated the use of mantle heat exchangers as means of degrading thermal stratification in

TES tanks and thereby enhancing the thermal performance of solar water heating systems.

Various researchers have also explored the possibility of using phase change materials (PCMs) as

heat storage media since they have much higher energy storage capacity and energy storage

density than water [2, 10-13]. For instance, Murray and Groulx [10] evaluated a PCM based TES

system and achieved about 44.3% increase in energy storage density as compared with water

based system. Similar investigation carried out by Delgado et al.[13] with PCM emulsion, also

achieved an increase of 34% in energy storage capacity as against water based TES system.

3

However, PCMs generally possess low thermal conductivity and hence poor thermal response

factor. For example the study by López-Navarro et al. [14] showed a declining thermal response

factor in a PCM storage tank after a few number of thermal cycling and was attributed to low

thermal conductivity of the PCM. Other general weakness in PCMs is instability in their melting

temperatures during long term storage operation. This phenomena was evident in a study by

Behzadi [15] when the melting temperature of the PCM shifted from 21 to 28℃ after 120

repeated thermal cycles. Apart from thermal instability issue, Chen et al. [16] also observed a

high level of thermal stress in a PCM storage tank as a result of considerable change in density

during phase transition period.

In order to overcome some of these problems the study was primarily focused on the

development of a novel high temperature microencapsulated phase change material (MEPCM)

for application in a compacted water saturated fixed bed system [12] as shown in Fig. 1. This

storage unit has the potential of achieving much higher effective thermal conductivity, enhanced

thermal stability and higher energy storage density with a relatively smaller physical storage size

than conventional water storage tanks.

4

2 Development of MEPCM

2.1 Material selection

Paraffin wax with a melting temperature of 48℃ was chosen as the base material for the

development of the MEPCM because of its relatively high latent heat capacity and phase change

temperature. Melamine-formaldehyde (MF) resins have been widely used as shell materials for

encapsulation processes due to their ability to be crosslinked at different

melamine/formaldehyde molecular ratios such as: 1:1.25-2 [17], 1:2.66 [18] and 1:2.5 [19]).

Therefore melamine with 99% purity level and 37 wt% formaldehyde solution was used as shell

monomers. Two types of emulsifiers i.e. Sodium dodecyl sulfate (SDS; Sinopharm Chemical

Reagent Co.,Ltd, China; purity 90%) and binary emulsifier Brij 30 (Aladdin, Japan) & Brij 58 (Sigma,

USA) were used to evaluate the effect of emulsifiers on the developed MEPCM. Citric acid

(Sinopharm Chemical Reagent Co.,Ltd, China) and sodium hydroxide (Sinopharm Chemical

Reagent Co.,Ltd, China) solution were used to control the pH values during the experimental

process. Ammonium chloride (purity 99.5%) also supplied by Sinopharm Chemical Reagent

Co.,Ltd. was used as a nucleation agent whereas deionized water was applied for dilution

processes. Table 1 shows a summary and composition of basic materials that were used for the

development of the MEPCM samples (MF-1, MF-2, MF-3, MF-4 and MF-5).

5

2.2 Fabrication of MEPCM

The fabrication of the MEPCM samples was based on in-situ polymerization method as well as

processes covering synthesisation of prepolymer solution, preparation of oil-in-water (O/W)

emulsion and formation of shells. The value of hydrophilic–lipophilic balance (HLB) [20] of

emulsifier is one of the most important factors for preparing a stable oil-in-water (O/W) emulsion.

Meanwhile, the average HLB value for the nonionic binary emulsifier can be calculated according

to the weight percentages and HLB values of the initial components [21]. Therefore Brij 30 and

Brij 58 were mixed together at an appropriate HLB value of 12 to prepare for the O/W emulsion.

As shown in Fig. 2, melamine (4.04g) and 37% formaldehyde solution (6.5g) were initially mixed

with 10ml deionized water at a stirring speed of 200 rpm. It was then heated up to 70°C and

maintained at that reaction temperature until the suspension became transparent. The

prepolymer solution was prepared at a pH value of 8.5–9 with 0.2wt% sodium hydroxide solution.

The next stage of the fabrication process involved mixing together the molten paraffin with

denoized water and the emulsifiers. The solution was then stirred at a speed of 7000rpm for

10min with a dispersion machine (IKA T18, Germany, see Fig.3) to obtain a stable O/W emulsion.

In order to fabricate the MEPCM, the capsules need to crosslink with MF polymer at a lower pH

value ranging from 3-5 as shown in the fabrication flow chart. For this reason the pH value of the

mixture was modified to 3–4 with 5wt% citric acid solution to initiate the crosslinking process.

Meanwhile the prepolymer solution was added to the O/W emulsion at a stirring speed of 300

6

rpm with a magnetic stirrer (type IKA HS-7, Germany, see Fig.3) for 4 hours at a temperature of

70°C. The end product was finally filtered, washed with water and then dried in an oven at a

temperature of 60°C for 20 hours.

3 Results and analysis

3.1 Evaluation of fabricated MEPCM samples

3.1.1 Scanning electron microscopy (SEM) analysis

A scanning electron microscope (SEM) by Sigma VP, Carl Zeiss Co. Ltd., was used to examine the

morphologies of the fabricated MEPCM samples. Since the shell material is a non-conductive

material, the samples were initially coated with 5 nm thick gold layer in order to increase their

electrical conductivity before the microscopy analysis was carried out. This procedure was in

accordance with a similar study carried out by Suzuki [22].

As shown in Fig. 4, the particles sizes of the samples were similar in sizes measuring about 2µm

but there were significant differences in their morphologies which were attributed to different

HLB values for the emulsifiers used for the encapsulation process. For instance the HLB value for

the anionic SDS emulsifier used for samples MF-1, MF-2 and MF-3 (Fig. 4a to Fig. 4c) was as high

as 40 which resulted in the agglomeration of the particles. On the other hand the MF-4 and MF-5

samples (Fig. 4d and Fig. 4e) achieved much better morphologies with a lower HLB value of 12 for

7

the binary emulsifiers (Brij 30 and Brij 58). In general emulsifiers are used for reducing interfacial

tension in O/W emulsions and to prevent coalescence by capsules adsorption onto the oil/water

interface in order to promote the formation of regular shapes of capsules during

microencapsulation process. It is obvious from the outcomes that the binary emulsifiers were

more effective than the SDS emulsifier due to the synergy of the two emulsifiers [23] hence.

3.1.2 Energy storage capacity, melting temperature and encapsulation

efficiency

A differential scanning calorimetric (DSC6220, SII Nanotechnology) equipment was used in

determining the enthalpies of fusion and melting temperature of the MEPCM samples in

accordance with ISO 11357 Standards under the dynamic testing method. The samples were

tested at atmospheric pressure and at a heating rate of 2 °C /min from 5℃ to 65℃.

As shown in Fig. 5 and in relation to the pure paraffin, sample MF-3 achieved the highest melting

enthalpy of 126 kJ/kg as against 57 kJ/kg, 84.3kJ/kg, 95.8 kJ/kg and 104 kJ/kg for MF-1, MF-2,

MF-4 and MF-5 respectively. With regard to their melting temperatures, there was no strong

evidence to suggest that the organic emulsifiers had any major effect on the samples since there

was only a slight reduction of between 0.08℃ to 0.95℃ as summarized in Tab. 2. The DSC results

were also used to determine the encapsulation efficiency which is defined as the ratio of the

actual core content of the microcapsules to the theoretical core content [24]. The percentages of

8

core material contents for the samples were therefore obtained as 29% for MF-1, 43% for MF-2,

65% for MF-3 49% for MF-4 and 54% for MF-5. Now based on the initial core/shell weight ratios

in Tab. 1, the corresponding encapsulation efficiencies were obtained as 44.23%, 65.18%, 97.42%,

74.07% and 80.41%.

Beside the morphology the type of emulsifiers also demonstrated some effects on the core

material content and encapsulation efficiency for the fabricated MEPCM samples. For instance,

the core material content for sample MF-4 (49%) was much higher than sample MF-1 (29%) due

to the effect of the binary emulsifier. However, comparison of MF-5 and MF-3 shows that the

core material content was reduced about 11% by the binary emulsifier, which was attributed to

the fact that some of the capsules in MF-5 were not fully encapsulated.

The results as summarised in Tab. 2 show that the nucleating agent (ammonium chloride) did

also influence the core material content and the encapsulation efficiencies of the MEPCM

samples. For instance the core material content and the encapsulation efficiency for MF-3 did

increase by more than 35% and 53% respectively with 0.25g of ammonium chloride as a

nucleation agent. Meanwhile, the core material content and the encapsulation efficiency of MF-5

also increased by 5%-6% thus showing the positive effect of nucleating agent when the binary

emulsifiers were used to prepare the MEPCM samples. This is based on the fact that ammonium

chloride as nucleating agent can effectively reduce the pH value [25] during the polymerization

reaction.

9

3.1.3 Thermal stability

Thermogravimetric (TG) technique can be used to analyse the thermal stability of a sample

material by measuring the amount of weight change of the material as a function of increasing

temperature in a controlled atmosphere of nitrogen, helium, air, or in vacuum, which exhibits loss

or gain of mass due to thermal decomposition, evaporation, oxidation or dehydration of the

sample [26]. The thermal stabilities of the MEPCM samples were therefore examined under

nitrogen gas protection covering a heating range of 50℃ to 500℃ and at a heating rate of

10℃/min. As presented in Fig. 6, the maximum weight loss temperatures for the samples were as

high as 283.7℃, 280.1℃, 279.7℃, 283℃ and 272.2℃ for MF-1, MF-2, MF-3, MF-4 and MF-5

respectively as compared with 238℃ for the pure paraffin. It should also be noted that in general

higher core material content does affect the thermal stability of capsules thus confirming why

MF-5 achieved the lowest weight loss temperature. Even though the shell material of the

samples has a lower melting point temperature of 180℃ than the corresponding weight loss

temperatures, it is far more higher than the temperature requirement for hot water (29–60℃)

[27] or space heating (30-65℃) [28] services in buildings. In addition the shell material can

protect the core material from the influences of the outside environment and to maintain

temperature stability for long term TES application. Above all the MF shell can accommodate

changes in volume during phase change process of the core material thereby decreasing the

thermal stress in TES tanks. Therefore, the long term operational thermal stability of the latent

10

heat storage media for the proposed TES unit can be improved by using the developed MEPCM.

3.2 Theoretical evaluation of proposed TES unit

As described in Fig. 1, the TES unit is similar to the concept adopted for enhancing energy storage

density and effective thermal conductivity in a study conducted by Darkwa et al. [29]. Therefore

by considering individual elements and their weight proportions the various percentages of

energy storage capacity may be calculated using Eq. 1 below as:

(1)

(2)

(3)

(4)

So that Eq.1 becomes:

(5)

Where

, and are the weight percentages of water, core material and shell material

respectively, %.

11

, and are the corresponding specific heat capacities, J/kg∙℃.

, , and are the corresponding densities, kg/m3.

is the latent heat of PCM, kJ/kg.

is the temperature difference, ℃.

is the weight percentages of MEPCM in TES unit, %.

is the core material content of MEPCM, %.

The effective thermal conductivity can also be calculated by considering the conductivity and the

volume fraction of each component as established by Thiele et al. [30] and reported by Felske [31]

in Eq. 6 as:

(6)

(7)

(8)

According to Eq.2-4, Eq.7 becomes:

12

(9)

By the same method,

(10)

Where

, and are the conductivities of water, paraffin and shell material, W/m∙K.

and are the volume percentages of water, shell material and core material, %.

Now using sample MF-3 and the data in Tab. 3, the energy storage density and the effective

thermal conductivity of the TES unit over a range of 40 to 60 ℃ with different weight percentage

of MPECM can be calculated and represented graphically in Fig. 7.

Analysis of the results show that energy storage density increases with weight percentage of the

MEPCM whilst effective thermal conductivity reduces. For instance if 50 wt% MEPCM is to be

used in the proposed TES unit, its energy storage density would increase from 84000 kJ/m3 to

128735 kJ/m3. It would also reduce the volume of the TES unit by about 34.7% in comparison

with a water storage tank of the same energy storage capacity. Even though the effective thermal

13

conductivity of (ke1) would reduce to 0.41 W/m∙K, it is still about 2 times that of pure paraffin. In

comparison with other studies, the effective conductivity of the developed MEPCM is much

higher than other thermal energy storage materials, such as lauric acid (0.148~0.150 W/m∙K) [10],

PCM emulsion (0.27 W/m∙K) [13], RT8 (0.20 W/m∙K) [14] and Rubitherm 21 (0.20 W/m∙K) [15].

Previous investigations also showed that thermal response time of TES units were poor due to

low thermal conductivity of the phase change materials. For instance the experimental results by

Murray and Groulx [10] showed that the time requirements for thermal energy charging and

discharging in a TES system were as long as 30 hours and 32 hours respectively. The thermal

response of the proposed TES unit is therefore expected to be much higher during thermal

energy charging/discharging cycling. There is however the need for the TES unit to be optimized

based on design and operational requirements.

4 Conclusions

The study was focused on the development of a high melting temperature MEPCM for solar hot

water storage system. Five MEPCM samples (MF-1, MF-2, MF-3, MF-4 and MF-5) have therefore

been successfully fabricated through in-situ polymerization method. Analysis of the results

showed that MF-3 achieved the highest enthalpy of 126 kJ/kg but there was no huge difference

in their melting temperatures which was ranged between 0.08℃ to 0.95℃. In general the overall

14

thermal stability of the samples was relatively high with MF-3 reaching 279.7℃. The

encapsulation efficiency was however influenced by the nucleation agent whilst the morphology

was also affected by the type of emulsifier since samples MF-4 and MF-5 achieved much better

morphology with the binary emulsifier than samples MF-1, MF-2 and MF-3 which were fabricated

with SDS emulsifier. In order to assess their thermal effectiveness, MF-3 was theoretically

evaluated in an integrated compacted fixed bed unit and found to be capable of contributing to a

system with higher energy storage density and relatively smaller physical storage size than water

based system. Even though the overall effective thermal conductivity of the unit was slightly less

than water, it was found to be about twice as high as most current PCM storage units. The

proposed TES unit is therefore expected to be thermally more responsive during thermal energy

charging/discharging process. Experimental evaluation is therefore being encouraged towards

achieving a workable prototype.

Reference

[1] J.F. Belmonte, P. Eguía, A.E. Molina, J.A. Almendros-Ibáñez, R. Salgado, A simplified method for

modeling the thermal performance of storage tanks containing PCMs, Applied Thermal Engineering,

95 (2016) 394-410.

[2] S. Seddegh, X. Wang, A.D. Henderson, Z. Xing, Solar domestic hot water systems using latent heat

energy storage medium: A review, Renewable and Sustainable Energy Reviews, 49 (2015) 517-533.

[3] M. Thirugnanasambandam, S. Iniyan, R. Goic, A review of solar thermal technologies, Renewable

and Sustainable Energy Reviews, 14 (2010) 312-322.

15

[4] H. Tanaka, T. Tomita, M. Okumiya, Feasibility study of a district energy system with seasonal water

thermal storage, Solar Energy, 69 (2000) 535-547.

[5] S. Furbo, E. Andersen, S. Knudsen, N.K. Vejen, L.J. Shah, Smart solar tanks for small solar domestic

hot water systems, Solar Energy, 78 (2005) 269-279.

[6] H.Ö. Paksoy, Thermal energy storage for sustainable energy consumption fundamentals, case

studies and design, Springer, Dordrecht :, 2007.

[7] M. Raisul Islam, K. Sumathy, S. Ullah Khan, Solar water heating systems and their market trends,

Renewable and Sustainable Energy Reviews, 17 (2013) 1-25.

[8] G.L. Morrison, A. Nasr, M. Behnia, G. Rosengarten, Analysis of horizontal mantle heat exchangers in

solar water heating systems, Solar Energy, 64 (1998) 19-31.

[9] J. Xu, R.Z. Wang, Y. Li, A review of available technologies for seasonal thermal energy storage, Solar

Energy, 103 (2014) 610-638.

[10] R.E. Murray, D. Groulx, Experimental study of the phase change and energy characteristics inside

a cylindrical latent heat energy storage system: Part 1 consecutive charging and discharging,

Renewable Energy, 62 (2014) 571-581.

[11] D.N. Nkwetta, F. Haghighat, Thermal energy storage with phase change material—A state-of-the

art review, Sustainable Cities and Society, 10 (2014) 87-100.

[12] M.K.A. Sharif, A.A. Al-Abidi, S. Mat, K. Sopian, M.H. Ruslan, M.Y. Sulaiman, M.A.M. Rosli, Review of

the application of phase change material for heating and domestic hot water systems, Renewable and

Sustainable Energy Reviews, 42 (2015) 557-568.

[13] M. Delgado, A. Lázaro, J. Mazo, C. Peñalosa, P. Dolado, B. Zalba, Experimental analysis of a low

cost phase change material emulsion for its use as thermal storage system, Energy Conversion and

Management, 106 (2015) 201-212.

[14] A. López-Navarro, J. Biosca-Taronger, J.M. Corberán, C. Peñalosa, A. Lázaro, P. Dolado, J. Payá,

Performance characterization of a PCM storage tank, Applied Energy, 119 (2014) 151-162.

[15] S. Behzadi, M.M. Farid, Long term thermal stability of organic PCMs, Applied Energy, 122 (2014)

11-16.

[16] S.-L. Chen, C.-L. Chen, C.-C. Tin, T.-S. Lee, M.-C. Ke, An experimental investigation of cold storage

16

in an encapsulated thermal storage tank, Experimental Thermal and Fluid Science, 23 (2000) 133-144.

[17] F. Cao, B. Yang, Supercooling suppression of microencapsulated phase change materials by

optimizing shell composition and structure, Applied Energy, 113 (2014) 1512-1518.

[18] J.K. Choi, J.G. Lee, J.H. Kim, H.S. Yang, Preparation of microcapsules containing phase change

materials as heat transfer media by in-situ polymerization, Journal of Industrial and Engineering

Chemistry, 7 (2001) 358-362.

[19] K. Hong, S. Park, Melamine resin microcapsules containing fragrant oil: synthesis and

characterization, Materials Chemistry and Physics, 58 (1999) 128-131.

[20] T.F. Tadros, Emulsion Science and Technology, Wiley, 2009.

[21] W.C. Griffin, Calculation of HLB valuess of Nonionic Surfactants, J. Soc. Cosmet. Chem, 5 (1954)

249-256.

[22] E. Suzuki, High-resolution scanning electron microscopy of immunogold-labelled cells by the use

of thin plasma coating of osmium, Journal of Microscopy, 208 (2002) 153-157.

[23] M.J. Rosen, Predicting synergism in binary mixtures of surfactants, in: M.J. Schwuger, F.H. Haegel

(eds.) Surfactants and Colloids in the Environment, Steinkopff, Darmstadt, 1994, pp. 39-47.

[24] H. Zhang, X. Wang, Synthesis and properties of microencapsulated n-octadecane with polyurea

shells containing different soft segments for heat energy storage and thermal regulation, Solar Energy

Materials and Solar Cells, 93 (2009) 1366-1376.

[25] I.S. Chuang, G.E. Maciel, Carbon-13 CP/MAS NMR study of the structural dependence of

urea-formaldehyde resins on formaldehyde-to-urea molar ratios at different urea concentrations and

pH values, Macromolecules, 25 (1992) 3204-3226.

[26] M.E. Brown, INTRODUCTION TO THERMAL ANALYSIS: Techniques and Applications, Springer,

Netherlands, 2001.

[27] W. Su, J. Darkwa, G. Kokogiannakis, Review of solid–liquid phase change materials and their

encapsulation technologies, Renewable and Sustainable Energy Reviews, 48 (2015) 373-391.

[28] D. Feldman, M.M. Shapiro, D. Banu, C.J. Fuks, Fatty acids and their mixtures as phase-change

materials for thermal energy storage, Solar Energy Materials, 18 (1989) 201-216.

17

[29] J. Darkwa, O. Su, T. Zhou, Development of non-deform micro-encapsulated phase change energy

storage tablets, Applied Energy, 98 (2012) 441-447.

[30] A.M. Thiele, A. Kumar, G. Sant, L. Pilon, Effective thermal conductivity of three-component

composites containing spherical capsules, International Journal of Heat and Mass Transfer, 73 (2014)

177-185.

[31] J.D. Felske, Effective thermal conductivity of composite spheres in a continuous medium with

contact resistance, International Journal of Heat and Mass Transfer, 47 (2004) 3453-3461.

18

Table 1: Materials for MEPCM fabrication

Table 2: Thermal properties of paraffin and MEPCM samples

Table 3: Thermophysical properties of TES components

Figure 1: Compacted MEPCM bed system for solar TES system

Figure 2: MEPCM fabrication procedure [17-19]

Figure 3: Experimental rig for MEPCM preparation

Figure 4: SEM examination of MEPCM samples

Figure 5: DSC analysis of pure paraffin and MEPCM samples

Figure 6: TG analysis of pure paraffin and MEPCM samples

Figure 7: Thermal profile of TES unit

19

Figure 1: Compacted MEPCM bed system for solar TES system

20

pH 8.5-9,200 rpm,

70℃

Prepolymer solution

70℃, 7000 rpm, 10

minutes

O / W Emulsion

200 rpm,

pH=3-5, 70 ℃, 4 hours

Melamine + Formaldehyde+ Deionized water

Water + Emulsifiers +

PCM

pH 9 to end crosslinking

MEPCMCentrifuge /

Wash

Nucleating agent

Drying

Figure 2: MEPCM fabrication procedure [17-19]

21

Figure 3: Experimental rig for MEPCM preparation

22

(a) MF-1

(b) MF-2

(c) MF-3

23

(d) MF-4

(e) MF-5

Figure 4: SEM examination of MEPCM samples

Figure 5: DSC analysis of pure paraffin and MEPCM samples

24

Figure 6: TG analysis of pure paraffin and MEPCM samples

25

Figure 7: Thermal profile of TES unit

Table 1: Materials for MEPCM fabrication

MEPCM

samples

Melamine

(g)

Formaldehyde

solution (g)

Emulsifier

(g)

PCM

(g)

Ammonium

chloride (g)

MF-1 4.04 6.5 SDS (0.5) 10 0

MF-2 4.04 6.5 SDS (0.5) 10 0.125

MF-3 4.04 6.5 SDS (0.5) 10 0.25

MF-4 4.04 6.5 Brij30 & Brij58 (0.17:0.33) 10 0

MF-5 4.04 6.5 Brij30 & Brij58 (0.17:0.33) 10 0.25

Table 2: Thermal properties of paraffin and MEPCM samples

Name Melting

temperature (℃)

H

(kJ/kg)

Core ratio

(%)

Encapsulation

efficiency (%)

Weight loss

temperature (℃)

Paraffin 48.34 194 —— —— 238.0

MF-1 48.25 57 29% 44.23% 283.7

MF-2 47.98 84.3 43% 65.18% 280.1

MF-3 47.66 126 65% 97.42% 279.6

MF-4 47.58 95.8 49% 74.07% 281.0

MF-5 47.39 104 54% 80.41% 272.2

Table 3: Thermophysical properties of TES components

Name Density (kg/m3) Cp (J/kg∙℃) Thermal conductivity (W/m∙K)

Water 1000 4200 0.62

Paraffin 800 2500 0.2

MF resin 1500 1200 0.35

26

Highlights

The nucleating agent increased core material content and encapsulation efficiency

The binary emulsifier did affect the morphology of the capsules

The MEPCM packed bed unit did increase the energy storage density